WO2008059294A1

WO2008059294A1 - Feed of high accuracy satellite positioning spiral and helical antennas

Info

Publication number: WO2008059294A1
Application number: PCT/GB2007/050695
Authority: WO
Inventors: Robin Granger
Original assignee: Roke Manor Research Limited
Priority date: 2006-11-16
Filing date: 2007-11-16
Publication date: 2008-05-22
Also published as: GB0622858D0

Abstract

A geodetic multi arm spiral antenna comprises an antenna feed, a processor and four antenna conductors. The antenna feed comprises a transmission line, the transmission line comprising four first conducting elements (15, 16, 17, 18) spaced from one another in a cavity (19); and a second conducting element (21), at least partially enclosing the first conducting elements within the cavity. The first conducting elements are aligned with one another and positioned such that the distance (27, 28, 29) between each first conducting element and the nearest point of the second conducting element is approximately equal.

Description

FEED OF HIGH ACCURACY SATELLITE POSITIONING SPIRAL AND HELICAL ANTENNAS

This invention relates to a geodetic antenna, in particular a multi arm spiral antenna and a helical antenna.

Typically geodetic antennas have been made as planar spirals. Planar spiral antennas have long been known to exhibit characteristics, such as wide bandwidth, broad half-power beamwidth and circular polarisation which make themparticularly useful in satellite communication and satellite navigation applications. A common configuration comprises a number of co-planar spiral conductors, often fabricated using printed circuit techniques, where each conductor, or 'arm', is fed independently and phased to give particular beam pattern properties. A conducting enclosure, or cavity backing, is often included on one side of the antenna which reduces radiation in that direction. The resulting radiation pattern is usually approximately hemispherical, with peak gain in the direction away from the cavity. The cavity can be filled with a low- loss dielectric, usually air, in which case the cavity is usually made approximately one quarter- wavelength deep; or the cavity is filled with a microwave absorbing material which facilitates much smaller cavities. In either case, and also in the case of a planar spiral with no cavity, the problem remains of how to excite, or feed, the conductors electrically, without significantly affecting the performance of the antenna by the proximity of the feed structure. A particularly difficult example of this problem is that of precision antennas for high- accuracy satellite positioning, often known as geodetic antennas. Such geodetic antennas are employed, for example, in global positioning system (GPS) systems used for surveying, tectonic plate monitoring, automated agriculture, etc. where position errors must be minimised and are often only a few millimetres. At this level of accuracy, the 'movement' of the phase centre of the antenna, i.e. of the point in space where electromagnetic energy appears to emanate, in the case of a transmitting antenna, can easily degrade the accuracy of the fix. Signals arriving at the antenna from different directions, but over the same distance, can suffer phase shifts which makes the distance appear to vary - effectively the phase centre 'moves' with angle of arrival. Phase centre stability is related to radiation pattern uniformity.

In some high-precision applications, the effects of the antenna are, in part, compensated for by post- processing of position data to allow for the variation in phase centre with elevation angle, since this angle can be calculated from knowledge of the relative positions of the satellites in question. However, the effects of azimuthal phase centre variation are not usually corrected, so the antenna must exhibit good phase centre stability with azimuth.

Spiral antennas are commonly fed using coaxial structures, typically with a characteristic impedance of 50ohms, where each centre conductor of the coaxial structure excites a single spiral arm, and the outer conductors of the structure are connected to each other and to a notional 'ground' conductor. The electric and magnetic fields around each centre conductor is independent of and not influenced by, the fields in any other centre conductor. An additional conductor can be included around the coaxial structures in order to improve the connection to the cavity and/or to the notional ground, as shown in US3381371. However, this design can result in an impedance mismatch, and thus reduced efficiency, between the feeds and the antenna arms. Theory predicts that each arm of a four arm, mode 1 spiral antenna should be around 133ohms, but in practice values of between 75 and lOOohms are more common. Construction and manufacture of coaxial feeds can also be complicated and expensive. An alternative to the coaxial feed is a parallel wire, balanced feed where simple wire conductors are connected to each of the spiral arms . An example of this is given in "Four Arm Spiral Antennas" by Corzine and Mosko, Artech House, 1990 and in US4609888. In this example the orthogonal mode feed is manufactured by printing onto a printed circuit board (PCB) and incorporates a tapered Klopfenstein impedance match. The feed is formed as a multi- trapezoidal slab of dielectric substrate, the upper part of which is substantially rectangular and provided with rectangular feed aperture inserts and the wires, in the form of planar rectangular feed strips, are positioned on each of the outer corners of the substrate. Each of the feed strips connects to an antenna arm. In such parallel wire transmission lines, however, the electric field is bound only very loosely between the conductors and can, therefore, interact with the near- field electric fields of the antenna itself, spoiling the radiation pattern. However, this is not an issue for the direction finding applications that are described in these documents, which have no need for shielding of stray fields the phase centre does not need to be as tightly controlled as for geodetic applications.

In accordance with a first aspect of the present invention, a geodetic multi arm spiral antenna comprises an antenna feed, a processor and four antenna conductors; wherein the antenna feed comprises a transmission line, the transmission line comprising four first conducting elements spaced from one ano ther in a cavity; and a second conducting element, at least partially enclosing the first conducting elements within the cavity; wherein the first conducting elements are aligned with one another and positioned such that the distance between each first conducting element and the nearest point of the second conducting element is approximately equal.

The present invention enables a multi- conductor antenna, to be connected to circuitry, such as a beam- forming network, with minimal effect on the electric field close to the antenna by providing the first conducting elements with a common shield with no other conductor between them. The conductors could have any shape cross- section, for example planar strips in the case of a printed circuit board construction, but the design is made more difficult because coupling between each of the conductors is harder to predict and control, so preferably, the first conducting elements comprise cylindrical wires.

Preferably, the processor controls the phase of each antenna conductor. It is necessary to modify the phases of the signals at each conductor before combining them in order to properly control the polarisation and radiation pattern properties of the antenna.

Preferably, the antenna conductors are arranged such that the antenna is substantially circularly symmetric. This further improves performance by reducing variation in gain with azimuth angle, which in turn increases the phase centre stability.

Preferably, the first conducting elements are tapered from one end to the other along the cavity.

The antenna conductors may be discrete components, but preferably, the antenna conductors comprise conducting strips disposed on a dielectric substrate.

This allows for low- cost manufacture of the antenna on a printed circuit board. Preferably, the spacing of the first conducting elements from one another varies along their length.

This enables the impendence to be adapted to achieve a required value at the antenna conductor end.

In accordance with a second aspect of the present invention, a geodetic helical antenna comprising an antenna feed, a processor and a helical antenna conductor, wherein the antenna feed comprises a transmission line, the transmission line comprising a plurality of first conducting elements spaced from one another in a cavity; and a second conducting element, at least partially enclosing the first conducting elements within the cavity; wherein the first conducting elements are aligned with one another and positioned such that the distance between each first conducting element and the nearest point of the second conducting element is approximately equal.

An example of a multi arm spiral antenna feed in accordance with the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a plan view of a four-conductor planar spiral antenna for use with the present invention;

Figure 2 illustrates a cross- sectional view of the antenna of Fig. 1 with a conductive cavity, containing an enclosed four- conductor feed structure according to the present invention;

Figure 3 is a perspective view of the feed structure of Fig. 2, at the point where it connects to the antenna; and,

Figure 4 is a cross- sectional view of the feed structure according to the present invention, showing the dimensions relevant for design and optimisation of the feed impedance.

Although the present invention could be applied to an antenna having at least two arms, the performance of a two arm antenna is not as good for geodetic applications as a four arm antenna. Similarly, although more than four arms could be used and the feed can be adapted to function with any number of arms this becomes much more complicated and is not the first choice in antenna design. Odd numbers of conductors are more complex to set the phasing for over medium or wide frequency bandwidths, so the antenna comprises four antenna conductors. Therefore, an example of the invention will be described with respect to a four- conductor, or four-arm, planar spiral antenna. The antenna is driven by appropriate driving circuitry incorporating necessary functionality, such as baluns and hybrids, for deriving or processing the signals applied to or present at each conductor. However, this is well known and not described in more detail here.

Between each antenna conductor, or arm and the corresponding connection to the processing circuitry is provided a conducting wire or strip, so for the example of a four arm antenna, there are four such wires aligned with one another and spaced apart by a small distance. The wires are enclosed within a conducting, cylindrical or conical tube which is aligned coaxially and positioned centrally such that the distance from each wire to the tube is equal. The tube is connected electrically to the antenna cavity enclosure. The wires may be parallel along their length, or taper away from top to bottom in the cavity to provide a different impedance at each end. The wires are shown in the example as solid cylinders, but they could also be planar strips, or other suitable shapes to achieve the desired impedance.

Figs. 1 and 2 show plan and cross- sectional views of an embodiment of the present invention A four- conductor planar spiral antenna comprises four conductors, or arms 1, 2, 3, 4 disposed on a dielectric substrate 5, the conductors having been fabricated by printed circuit techniques to give a spiral effect. At the centre 6 of the dielectric substrate is a connection 7, 8, 9, 10 for each arm to a feed structure. In this example, each of the conducting strips tapers to a point 11, 12, 13, 14 at its end furthest from the centre. The connections 7, 8, 9, 10 are made between each of the arms and a number of conducting feed wires, or printed strips 15, 16, 17, 18 from processing circuitry. The feed wires are arranged in a cavity 19 of a cylindrical conducting enclosure 20 and are aligned substantially parallel to one another and their long axis is normal to the plane of the dielectric substrate of the spiral antenna. Typically, the connections are made by soldering, welding or using conductive epoxy, but any suitable method can be used.

The cylindrical conducting enclosure 20 forms the cavity 19 behind the antenna in order to confine the directions of maximum radiation to the antenna's upper hemisphere. The cavity 19 may be filled with a high dielectric, or lossy, material to reduce overall size for the same performance effect.

Around the feed wires 15, 16, 17, 18 is provided a conducting cylindrical tube, or sleeve 21 which extends upwards from, and is electrically- connected 22, to the enclosure 20. The sleeve 21 is aligned coaxially with the feed wires and positioned with respect to the feed wires such that the distance (26, 27, 28) between each wire and the sleeve is substantially equal. The upper extent of the sleeve does not touch the spiral arms 1, 2, 3, 4 or their connectors, resulting in a gap 23. The sleeve prevents stray electric or magnetic fields from the wires from interfering with the antenna electric or magnetic fields. The associated circuitry, including a processor 24, for deriving or processing the signals applied to, or present at, each conducting strip 1, 2, 3, 4 can be connected at any distance 25 beyond the lower extent of the cavity and will not unduly affect the electric fields within the cavity. Typically the connections can be made in a similar manner to the connections 7, 8, 9, 10 with the antenna, as described above.

A perspective view of the feed structure in the region of the connection 7, 8, 9, 10 to the antenna is given in Fig. 3. For clarity, the antenna conductor strips 1, 2, 3, 4 and substrate 5 are not shown. The feed wires 15, 16, 17, 18 are spaced from one another in a non-conducting region 26 and in pairs 15, 17 and 16, 18 at right angles to one another. The cylindrical sleeve 21 surrounds the feed wires except at the connection end.

Fig. 4 illustrates a cross- sectional view of the feed structure, including those dimensions relevant to the design and optimisation of the feed for a particular impedance or impedance transformation. The values of the various parameters separation between each feed wire of a twin line pair, s; diameter of each feed wire, dr, sleeve inner diameter, dϊ, and the gap 23 are determined by analysis, or computer simulation based upon microwave principles so as to match, principally, the impedance of the feed to that of the antenna arms. For values of d₂ which are much larger than the sum of s and twice di, the analysis of parallel conductors in a dielectric, depending on the relative permittivity of the region 26 in which the wires are positioned, can be applied. For smaller values of J₂, the analysis is more complicated as it must include the effects of the sleeve. It is desirable to minimise the overall outer diameter of the feed structure in order to minimise the effect of the feed on the antenna, especially at high frequencies. The conductors of a four- arm, mode 1 spiral antenna typically have impedance of the order of 75 to 100 ohms per conductor. Feeding such antennas using 50 ohm coaxial cable is common, but can clearly result in an impedance mismatch. The enclosed, multi- conductor feed structure presented here is generally suited to higher impedances than 50 ohms, and its mechanical simplicity means that it is relatively easy to design to a particular impedance for a good match. By varying the design parameters s, di, d₂ along the length of the feed, a different characteristic impedance may be produced at the driven end of the feed in order to match the impedance of the driving circuitry of the antenna. The radiation pattern of the antenna needs to be highly circularly- symmetric about an axis normal to the plane of the antenna, so it is desirable that the mechanical structure of the electrical parts of the antenna are also circularly- symmetric, including the feed. The inclusion of the sleeve 21 improves the circular symmetry of the electric fields near to the antenna elements 1, 2, 3, 4 which would otherwise be influenced by the feed conductors 15, 16, 17, 18. For spiral antenna designs which have an inherently high degree of circular symmetry, e.g. multi-turn, multi-arm Archimedean spirals, this can result in a significant improvement in azimuthal radiation pattern uniformity and phase centre stability.

Other types of antenna can be used for geodetic applications, such as a helical antenna which has a constant radius and is extruded up into a helix.

The present invention is able to transmit signals between the antenna and processing circuitry in an efficient manner, with minimum distortion or attenuation to the signals, and with minimum effect on the circular- symmetry of the radiation pattern. It provides an enclosed, multi- conductor transmission line structure, suitable for feeding multi- arm spiral antennas, or helical antennas which has a low impact on the antenna's near- field and a better impedance match than traditional coaxial feeds. Another advantage that the invention brings is a reduction in mechanical complexity and the associated costs and assembly time compared to a feed structure based on multiple coaxial transmission lines. A further advantage that the invention brings is the opportunity to incorporate a simple tapered impedance match if necessary, at a very low cost, by spacing the conductors further apart at one end of the feed than the other. This can be done simply by fixing the top and bottom of the lines, so that they are angled relative to the axis perpendicular to the antenna plane, e.g. by spacing fixing holes in the base of the enclosure 20 and the dielectric substrate 5 at different distances away from the axis.

Claims

1. A geodetic multi ami spiral antenna comprising an antenna feed, a processor and four antenna conductors; wherein the antenna feed comprises a transmission line, the transmission line comprising four first conducting elements spaced from one another in a cavity; and a second conducting element, at least partially enclosing the first conducting elements within the cavity; wherein the first conducting elements are aligned with one another and positioned such that the distance between each first conducting element and the nearest point of the second conducting element is approximately equal.

2. An antenna according to claim 1, wherein the first conducting elements comprise cylindrical wires.

3. An antenna according to claim 1 or claim 2, wherein the processor controls the phase of each antenna conductor.

4. An antenna according to any preceding claim, wherein the antenna conductors are arranged such that the antenna is substantially circularly symmetric.

5. An antenna according to any of preceding claim, wherein the first conducting elements are tapered from one end to the other along the cavity.

6. An antenna according to any of claims 1 to 5, wherein the antenna conductors comprise conducting strips disposed on a dielectric substrate.

7. An antenna according to any preceding claim, wherein the spacing of the first conducting elements from one another varies along their length.

8. A geodetic helical antenna comprising an antenna feed, a processor and a helical antenna conductor, wherein the antenna feed comprises a transmission line, the transmission line comprising a plurality of first conducting elements spaced from one another in a cavity; and a second conducting element, at least partially enclosing the first conducting elements within the cavity; wherein the first conducting elements are aligned with one another and positioned such that the distance between each first conducting element and the nearest point of the second conducting element is approximately equal.